1. Field of the Invention
The present invention relates to a system and method for effectively and efficiently retransmitting data frames, which were inadequately received by a receiver, back to the receiver for combination with the inadequately received data frames to increase gain at the receiver. More particularly, the present invention relates to a system and method for transmitting data in data frame format using an R-Rake retransmission technique which employs a blind identification scheme for identifying the retransmitted data frames to be combined with previously transmitted data frames at a receiver, thus eliminating the need to transmit a signaling message to the receiver for identifying the data frames to be combined.
2. Description of the Related Art
Wireless communication systems, such as satellite-based or terrestrial-based communications systems, as well as wire-line communications systems, such as public switched telephone networks (PSTNs) often employ a retransmission protocol to increase the reliability at which the transmitted data is received by a receiver. Retransmission protocols are especially useful in communications networks which transmit data in packet or frame format in a non-delay sensitive manner. For purposes of this disclosure, the terms “data packet” and “data frame” will each be referred to simply as a “data frame” or “frame”.
One known data retransmission protocol is referred to as an automatic request retransmission protocol (ARQ). According to ARQ, a transmitter first segments the data to be transmitted into a data frame, and attaches cyclic redundant code (CRC) bits to each frame prior to transmission. Each frame is embedded with a sequence number. Upon receiving each transmitted data frame, a receiver checks the CRC bits. If a frame fails to pass the CRC check, the receiver discards the failed frame and requests retransmission of the failed frame based on the missing sequence number.
Framing and sequence numbering may be accomplished at the layer above ARQ. In that case, the ARQ protocol still needs to record all the frames transmitted for satisfying potential retransmission request. These frames have to be retained until an acknowledgement is explicitly or implicitly given from the receiver, or until time-out as pre-defined by the protocol. However, for the purposes of this disclosure, the distinction between ARQ and the layers about ARQ is not significant, and therefore they will be referred to simply as an ARQ layer. This transmission and retransmission will continue until a valid frame is received, or until the limit on the number of permitted retransmissions which has been set up based on the quality of service required for this particular communication service has been reached.
According to a technique referred to as R-RAKE, instead of discarding the frame that fails the CRC check, the receiver retains the soft input from the communication channel. The retransmission protocol then requests a retransmission of the failed data frame, and the retransmitted data frame is soft combined with the previously received data frame. The soft combined frame is then decoded. For static channel systems, such as wire-line communication systems and some types of satellite communication networks, this combining of data frames doubles the signal to noise ratio, thus providing a gain of 3 dB for the transmitted data frame.
For mobile communications systems, channels typically experience fading. However, because the retransmitted data frame is transmitted at a later time than the originally transmitted data frame, additional spreading gain is provided. Depending on the speed of the vehicle in which the mobile unit is present, as well as the overall condition of the channel, the diversity gain is typically within the range of 2-4 dB. The additional gain provided by the R-Rake technique can therefore increase the gain to about 5-7 dB in these types of systems.
In spite of the enormous gain provided by the R-Rake technique, the conventional R-Rake technique is often deemed impractical due to the following reasons. First, because the R-Rake technique can be utilized most efficiently only if the retransmitted frame is soft combined with the previously received data frame, the conventional technique typically requires the use of a large amount of memory to buffer the failed data frames.
Secondly, the transmitters in most communications systems typically will not stop transmitting data frames to wait for a retransmission request from the receiver. Rather, the transmitter will keep transmitting new data frames to the receiver, and any retransmitted data frame is therefore mixed into the stream of newly transmitted data frames. Accordingly, the receiver must be capable of identifying the retransmitted data frames that should be combined with the failed data frames stored in the buffer. This process is referred to as “frame identification”.
The conventional R-Rake technique uses a message to identify the retransmitted frame. Prior to retransmitting a frame, the transmitter transmits this message to schedule a retransmitted slot at the receiver. As can be appreciated, this messaging technique requires a large amount of signaling overhead, and overall performance will suffer if any such message is lost. Furthermore, soft combining must be done at the physical layer.
On the other hand, the message may be processed in a higher layer of the protocol. However, this may incur further delay and extra network traffic. For instance, in a cellular network, physical layer processing is typically done at the Base-station Transceiver Subsystem (BTS) and higher layers are typically processed at the Base Station Controller (BSC). Hence, in order to process a message in a higher layer in a cellular network, the message would have to be sent from the BSC to the BTS, which incurs additional delay and network traffic. Due to these signaling drawbacks, the conventional R-Rake technique is deemed impractical.
Accordingly, a need exists for a system capable of effectively and efficiently retransmitting data frames to a receiver for combination with corresponding failed data frames at the receiver to increase gain at the receiver. Also, a need exists for a system capable of effectively using an R-Rake technique for retransmitting data frames without incurring the above drawbacks associated with the conventional R-Rake technique.